Conventionally, a sphygmomanometer is applied indirectly to an appendage such as a finger or an earlobe because it is convenient. For example, U.S. Pat. No. 4,406,289 discloses the use of a servo balance technique. Another example is U.S. Pat. No. 4,597,393 which discloses a method of calculating and estimating the lowest blood pressure by observing the linearity of the blood vessel and the associated peripheral organization. Further examples are U.S. Pat. Nos. 3,104,661, 3,920,004 and 4,437,470 which disclose the use of a plurality of cuffs or sensors. Also, a sphygmomanometer may include an optical means which senses a change in the vessel diameter or volume due to the applied pressure and the internal blood vessel pressure. The optical means converts the change in blood pressure to an electric signal. Using the optical means requires the use of a cover to eliminate the influence of external light, solar light, or the like and in addition requires a place where little external light is present and/or the external light is stable. If these requirements are not considered, an error will result from the sphygmomanometry. For example, when the average blood pressure is to be measured using a signal processor shown in FIG. 1, a light emitting element drive circuit Dr activates a light emitting element Ea to emit intermittent light. The output signal from a photosensitive element Da is amplified by an amplifier A4. The intermittent components contained in the amplified signal are eliminated by a filter F1 in order to obtain an output signal having a continuous waveform. The photosensitive element Da receives a quantity of varying light which depends on a change in diameter of the blood vessel. The output signal from the photosensitive element Da is an electric signal which corresponding to the change in the diameter of the blood vessel. As shown in FIG. 2(a) if the pressure PO of a bladder B (FIG. 1) is temporarily increased higher than the highest vessel pressure and then decreased gradually, the output signal from the signal processor changes as shown in FIG. 2(b). In this case, the blood pressure corresponding to the maximum amplitude of the output signal is handled as an average blood pressure. Reference characters F and A in FIG. 1 denote a finger and a blood vessel, respectively.
In the U.S. Pat. No. 4,406,289, the device used is large in size, and requires a long time to attach the device and to adjust it, so that the device is not suitable for an application in which the measurement of blood pressure is instantaneously desired. U.S. Pat. No. 4,597,393 has several problems such as (a) producing an error due to an assumption that the elasticity of the blood vessel and its associated peripheral organization is linear, (b) producing an error due to an increase in the quantity of data due to the calculation of a small waveform area, and (c) producing an error due to reducing the quantity of data, etc. In U.S. Pat. Nos. 3,104,661, 3,920,004, and 4,437,470, it is hard, to attach two or more device elements. Pressurization cannot be allowed because it may cause an error in the measurement by a sensor on the distal side. In addition, it is impossible to determine the lowest blood pressure or a method for the determination is unclear. In the sphygmomanometer which uses an optical means, the filter cannot eliminate external light similar in frequency to the pulsating wave. Thus, all the prior art, in which light emitting elements are disposed at opposing positions on a portion of a human body to sense a change in the vessel diameter as a change in the received quantity of light which is then changed to an electric signal which is in turn processed to measure the blood pressure, will cause an error in the sphygmomanometry.